Foden, who had switched to full-back for Sale with considerable success before a shoulder injury stalled his progress earlier this term, will join Saints in the summer.
Northampton, champions-elect of National League One, are set to make an immediate Guinness Premiership return after being relegated last year.
Saints have already recruited Foden's Sale colleague Chris Mayor, in addition to Ulster back-row forward Roger Wilson and London Irish centre Nils Mordt.
Sharks rugby director Philippe Saint-Andre said: "I am disappointed that Ben will not be staying with us, and I wish him well in his career.
"I have two international scrum-halves in Richard Wigglesworth and Dwayne Peel (next season), and was unable to promise Ben games at scrum-half, which is his chosen position."
And Sale chief executive James Jennings added: "We are naturally disappointed to lose a player of Ben's ability to another club.
"Ben has come through the ranks at Sale Sharks.
"Ben sees his future playing in the scrum-half position, whereas Philippe Saint-Andre and several other notables in the game believe that since Ben moved to full-back at the start of this season he may well have been involved in the Six Nations had he not damaged his shoulder playing sevens (for England) in South Africa."
Foden, 22, is expected to make a playing return this season, with Sale making a strong push for silverware in the Premiership and European Challenge Cup.